The Rheology, Microstructure and Processing of Complex Fluids

Introduction

Oil derived products are an integral part of Western lifestyle and it is difficult to imagine a world without them. The rate of discovery of new oil fields is now less than the rate of consumption of oil and as the search for viable alternatives continues efforts are being concentrated on improving the recovery of existing reserves.

One method of improved oil recovery is hydraulic fracturing, whereby the porosity of the rock surrounding the well is increased by the placing of proppant (sand particles) in cracks generated by pressurized fluids. In a hydraulic fracturing treatment a fluid is pumped into the oil well at high pressure. This creates cracks in the reservoir rock, into which the fluid flows. The same fluid is now pumped containing small, suspended particles (proppant), which it transports into the fracture. When the pressure is reduced the oil washes the fluid used in fracturing away, leaving behind the proppant wedging open the cracks. The ideal fracturing fluid has a low pressure drop in pipe flow, has adequate viscosity to carry proppant and degrades after the fracture has closed, leaving no residue. It is the rheology, microstructure and processing of these complex fluids on which this project concentrates.

Summary

The objective of this work was to characterise experimentally the rheology and processing properties of two fluids used in the hydraulic fracture of oil wells and to examine the applicability of models in the literature to them. The two fluids were also studied with air or particulate inclusions. The aim of this work was to improve the currently incomplete understanding of the flow of these fluids and their role in the hydraulic fracturing of oil wells.

Hydraulic fracturing is a method to increase the amount and rate of oil production from an oil well. Two fluids that can be used in this process are guar gum, which is a naturally occurring polysaccharide, and a wormlike micelle solution. The wormlike micelle solution is a blend of erucyl bis (2-hydroxyethyl) methyl ammonium chloride (an amphiphilic molecule derived from rape seed oil) with iso-propanol and brine.

Experimental work was carried out to rheologically characterise these two fluids using a controlled strain rheometer (a Rheometrics ARES), a controlled stress rheometer (a Bohlin) and a Multi-Pass Rheometer (a specially developed two-piston capillary rheometer). The Multi-Pass Rheometer experiments provided shear data at shear rates up to 80,000 s-1. Optical observations using direct and polarised light were also made on the fluids, with and without inclusions, in rheometric and complex geometries.

Fluids

Guar Gum

Guar gum is an edible, plant seed polysaccharide extracted from the guar gum bean plant by a series of crushing, sifting and grinding stages. Guar gum has many uses in the food, textile, paper and mining industries, in addition to its use in the hydraulic fracturing of oil wells, where it has been used since the 1960s. Guar gum has a linear mannose backbone randomly substituted with galactose side branches in a ratio of 1.6:1 (figure 1). Guar gum is studied in aqueous solution at 0.75 wt% and 1.5 wt%.

Figure 1. The structure of guar gum.

Wormlike Micelle Solution

The wormlike micelle solution studied is an aqueous solution of 3 wt% erucyl bis (2-hydroxyethyl) methyl ammonium chloride (derived from rape seed oil) with iso-propanol and 3 wt% potassium chloride. The system is marketed commercially by Schlumberger as ClearFRAC. The molecular structure (figure 2) consists of a hydrophobic tail and a hydrophilic quaternary ammonium head group. The amphiphilic molecules form wormlike micelles (molecular aggregates) in brine (figure 3). The high viscosity is due to entanglements of the wormlike micelles (figure 4).

Figure 2. The structure of the surfactant molecule.

Figure 3. Schematic diagram of a wormlike micelle.

Figure 4. Schematic diagram of entangled wormlike micelles.

The wormlike micelle solution has several advantages over guar gum in the hydraulic fracturing process. Namely, it is easier to prepare, produces less waste, causes less damage to the rock formation and leaves no residue when the well is produced (close to 100 % of the wormlike micelle solution flows back, compared to 35-40 % of a guar gum solution). The main disadvantages of the wormlike micelle solution in hydraulic fracturing treatments are the upper temperature limit at which it is effective and its high cost. Foamed wormlike micelle solution has been used in fracturing to reduce its cost (Chase et al, 1997).

Experimental Techniques

The first stage of the project was to examine the rheology of the fluids in a Rheometrics controlled strain rheometer and the Cambridge MultiPass Rheometer. Experiments were also carried out using a controlled stress Bohlin rheometer at Schlumberger Cambridge Research. The controlled strain rheometer was used to measure a strain sweep, which allowed a linear viscoelastic region to be identified. A frequency sweep was then performed in the linear viscoelastic region and relaxation after step strain was used to probe both the linear viscoelastic region and the non-linear viscoelastic region. The flow curves were obtained using all three rheometers, to achieve a reproducible data covering a large range of shear strain rates.

Direct optical observations and flow induced birefringence of the flow of the fluids in rheometric and complex flow fields have been made. The fluids were also investigated optically with air and particulate inclusions. The final stage of the project was to use the commercial finite element package Polyflow to model the fluid flow.